This invention relates generally to system control and more particularly to a system and method for self-tuning feedback control of a system.
Modern systems perform precision operations and must be robust against various disruptions such as noise and disturbances. To combat these disruptions, run-to-run control methods attempt to compensate for system errors. These run-to-run control methods often rely on techniques to model noise and disturbances affecting the system. Run-to-run control methods are often xe2x80x9clearning,xe2x80x9d in that error corrections are based on past process mistakes. These learning methods may suffer instability or poor performance while detecting and compensating for errors.
Wafer fabrication is a process particularly suited to run-to-run control, since wafer fabrication requires precise alignment between a photomask and a wafer. Alignments during fabrication are susceptible to many errors, such as variations in the optical path of an alignment sensor due to thermal effects or overlay shifts caused by maintenance and parts renewal. To combat process errors, photolithography steppers typically allow an operator to specify offset corrections. These offsets adjust the target location on the wafer to compensate for process errors. However, controlling these systems is a time consuming and expensive process, and problems in run-to-run control systems can introduce instability and result in producing defective devices. Thus there is a need for effective run-to-run control without sacrificing system stability.
In accordance with the present invention, a system and method for self-tuning feedback control of a system is provided which substantially eliminates or reduces disadvantages and problems associated with previous systems and methods. According to one embodiment of the invention, a method for controlling a system determines a nominal gain of the system, a maximum gain of the system, and a sequence of error measurements of the system. Based on the nominal gain, the maximum gain, and the sequence of error measurements, the method determines a stable sequence of tuning gains. The method tunes the system using the stable sequence of tuning gains.
In accordance with another embodiment of the present invention, a self-tuning photolithography system includes a stepper that aligns a target of a photolithography overlay based on an alignment command. The system further includes a sensor that generates an alignment measurement and a metrology device that detects an error of the stepper. This error comprises a high frequency component induced by an unknown noise and a low frequency component induced by an unknown disturbance. The system also includes a tuner that determines a stable tuning gain based upon the error, a nominal gain for the stepper, and a maximum gain for the stepper. In addition, the system includes a feedback controller that determines a feedback offset based upon the error and the stable tuning gain. The system further includes a stepper alignment controller that generates the alignment command based upon the alignment measurement and the feedback offset.
The invention provides a number of technical advantages. The present invention controls a response to a disturbance without a priori knowledge of the disturbance or noise affecting the system. This provides control without the need for modeling the perturbations affecting the system. In addition, the present invention self-tunes to maintain system stability while responding to disturbances masked by noise. Thus the system combats process errors without disrupting the process and without frequent maintenance of a control device. The invention also filters higher frequencies, ensuring that controlled processes are robust in the presence of noise, including metrology noise.
Incorporating the present invention into a photolithography process also provides technical advantages. The invention controls the photolithography process using a stable sequence of tuning gains that filter out responses to noise and reduce errors introduced by feedback control. This automated control reduces maintenance overhead and the number of rejected devices produced, and thus increases efficiency and decreases production costs. In a wafer fabrication plant, each run is worth thousands of dollars. Therefore, the present invention""s stability provides critical control without introducing costly errors.
Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, descriptions, and claims.